This invention relates to a differential amplifier device which includes two transistors as a differential pair and, in particular, to a differential amplifier device having a current-to-voltage converter which converts, into a voltage signal, a current signal outputted from one of the two transistors.
The differential amplifier, a main part of the differential amplifier device, produces a current signal corresponding to difference between two input voltages. Generally, the differential amplifier comprises a differential pair, and two transistors having a particular property. Now, description will be made about a case in which two npn-type transistors are employed as a differential pair. Power supply voltage is applied to a collector terminal of one of the transistors of the differential pair. In this condition, two input voltages are supplied to base terminals of the two transistors, respectively. In the case where the other transistor of the differential pair is an output transistor, a collector of the output transistor receives a collector current having a current value dependent on the difference between the two input voltages as an output current signal which the differential amplifier produces.
In order to obtain a voltage signal, which changes in response to the difference between the two input voltages, by using the output current signal which the differential amplifier produces, the differential amplifier device requires a current-to-voltage converter to convert a current signal to a voltage signal.
A conventional differential amplifier device comprises a current-to-voltage converter only consisting of a resistor. The resistor comprises two terminals. One of the terminals is connected to a collector terminal of an output transistor of the differential amplifier, and another terminal is applied with a reference voltage. In accordance with this current-to-voltage converter, the output current signal from the differential amplifier goes through the resistor. At that time, a voltage drop from the reference voltage occurs at the resistor. The conventional current-to-voltage converter generates a voltage signal by utilizing the voltage drop.
However, the conventional differential amplifier device has the following problem, that is, a collector current value of the output transistor is limited.
Generally, a reference voltage is lower than a power supply voltage because stepping down the power supply voltage produces the reference voltage. If the difference between the reference voltage and the source voltage is large, a value of the output current signal does not reach a theoretical value which depends on the difference between two input voltages but is limited to a level lower than above-mentioned theoretical value.